Rotary shafts of rotary pumps and other devices are usually supported for rotation using bearings. Most of these bearings fail long before their theoretical design life is over. It has been found that the major reasons for such premature bearing failures are loss of oil and oil contamination.
Oil contamination may occur from several sources. First, during each start and stop cycle the bearing housing admits a substantial amount of exterior air. The bearing housing of a typical pump with a 1.875" diameter shaft is designed to operate at 185.degree. F. As the pump is actuated and brought to operating temperature, the air within the bearing housing expands to such a degree that approximately 27.5 cubic inches of air will be expelled. Upon shutdown, the bearing housing cools, drawing in an equivalent amount of exterior air. This air is often highly saturated with moisture, and may be further contaminated with ambient particles or with fumes from various industrial processes.
Second, whenever the pump is shut down and exposed to temperatures near the dew point, moisture in the air condenses on the walls of the pump and eventually mixes with the oil or other lubricant. Oil will dissolve up to 0.5% of water by volume, with additional water settling to the bottom of the sump. Various additives in commercial oils (such as emulsifiers) will increase the amount of dissolved water. Once trapped by the oil, the water can not evaporate when the pump is reheated. Furthermore, the metals used in most bearing housings effectively make them into radiators which will cause moisture condensation within the pump even when the ambient air has not reached its dew point.
Finally, since the bearing housing is an isolated lubrication system, any foreign material that enters the bearing housing tends to accumulate in the oil. Such contaminants may include water, metal fines, or pumped fluid migrating down the shaft. These contaminants do repeated damage to the bearings over time. As the quantity of contaminants builds up, the rate of bearing damage accelerates.
Various studies have indicated that very small quantities of water present in the lubricant will cause rapid deterioration of the bearing and premature bearing failure. This is further compounded by vapors dissolving into the oil, particularly corrosive vapors.
In analyzing bearing failures and after conducting extensive research into various theories of bearing failure, applicant has concluded that the best way to extend bearing life is to provide a clean, filtered source of oil for lubrication, and to environmentally protect the bearings from the introduction of moisture, fumes, and other contaminants.
Various devices are currently known for either isolating a lubrication system from its ambient environment, or for purifying lubricant to rid it of contaminants that may have entered the system. Among these devices are bearing isolators, oil purification systems, and mist lubrication systems.
Bearing isolators or "labyrinths" are intended to overcome the short life of so-called "lip seals". Bearing isolators retain oil within the bearing housing, and prevent gross introduction of moisture, dirt, and other contaminants through the shaft opening. Bearing isolators do not effectively address the problem of moisture and fume contamination, nor do they provide any oil filtration. In fact, there are indications that bearing isolators may actually increase moisture contamination by providing an open passage to allow free flow of potentially contaminated air into and out of the bearing housing.
Oil purification systems are relatively complex, and involve the following steps:
1. Draining the oil from the bearing housing at predetermined intervals;
2. Filtering the oil to remove solids;
3. Heating the oil to a high temperature in a near-vacuum, and dropping the oil onto a spinning disk to remove dissolved water; and
4. Returning the oil to the bearing housing.
Oil purification systems require a significant capital investment which can be justified only in large facilities with many pumps. Oil filtration lowers the average level of contaminants, but does so only after the contaminants have entered the bearing housing.
Oil mist systems employ nozzles or other apparatus to atomize the oil, thus removing dissolved water from the oil. Although several theories have been advanced to account for increased bearing life attained with oil mist systems, the major advantages appear to be that a constant supply of clean oil is introduced into the bearing housing, and that positive pressure is introduced into the bearing housing, thus preventing the introduction of potentially contaminated ambient air during periods of shutdown.
Oil mist systems generate a significant degree of oil vapors, which must be vented to the outside atmosphere. This not only restricts the oil mist systems to outdoor use, but has prompted the Environmental Protection Agency to target oil mist systems as a pollution source that should be eliminated, as soon as a viable alternative technology is available.
It is therefore apparent that the need exists for a bearing lubrication system that will provide a supply of lubricant that is free of moisture and other contaminants, and can be operated in an environmentally sound manner.